The present invention relates to a process for producing a heat-resistant optical element comprising a liquid crystalline polymer and useful in the fields of display, opto electronics and optics.
Liquid crystalline polymers are optically anisotropic and can take various molecular orientation structures as compared with polymers used commonly, and it is known that they can be utilized as various optical elements by fixing such structures thereof. Particularly, an optical element having a helical (twisted) structure in the interior of film can be attained by only an optical element using a liquid crystalline polymer. Thus, it is possible to produce optical elements having characteristics which cannot attained by conventional optical elements. It is also possible to form optical elements on various substrates by a method wherein a liquid crystalline polymer layer is oriented on an orientating substrate, and fixing, it is transferred onto a light transmitting substrate.
As optical elements using a liquid crystalline polymer, the present inventors have proposed a viewing angle compensator for liquid crystal display having a helical (twisted) structure in the interior of film (Japanese Patent Laid Open No. 87720/1991) and an optical rotator (Japanese Patent Application No. 126962/1990). These optical elements exhibit a high performance which has not been attained by the other conventional optical elements, and those formed on polymer films are light in weight, thin and highly flexibl and thus are epoch-making optical elements also having the characteristics peculiar to the polymer films. At high temperatures, however, although those optical elements are stable as long as an external force is not applied thereto, there occurs a flow of the liquid crystalline polymer layer once even a slight external force is exerted thereon because the liquid crystalline polymer possesses fluidity, resulting in that the orientation structure of the liquid crystalline polymer is destroyed and so it is impossible to maintain a predetermined optical performance. Consequently, restrictions have been placed on working environmental conditions and secondary processing conditions for the optical elements as well as conditions for mounting the optical elements to devices. Therefore, in the case of the conventional optical elements using liquid crystalline polymers, it is difficult to apply them to display elements to be mounted on vehicles and projection type liquid crystal displays for which is required a high heat resistance in their working environment. Further, in the manufacturing process for optical and display devices using optical elements containing liquid crystalline polymers, the maximum temperature in the process is limited.
It is the object of the present invention to solve the above-mentioned problems of the prior art.
Having made extensive studies for overcoming such problems of optical elements using liquid crystalline polymers, the present inventors found out that such problems were attributable to the necessity of using a liquid crystalline polymer in a state of high fluidity to orientate the polymer during the fabrication of an optical element using the liquid crystalline polymer and another necessity of keeping the polymer's fluidity low after the end of the orientation. More particularly, according to the finding of the present inventors, if heat treatment is performed during the orientating treatment for the liquid crystalline polymer or after fixing of the orientation obtained or after transfer, it is possible to orientate the liquid crystalline polymer, reduce the fluidity thereof while maintaining the orientation structure and impart heat resistance no the optical element.